Geochemical prospecting



April 27, 1965 c. s. HALL, JR., ETAL 3,180,983

GEOCHEMICAL PROSPECTING Filed Dec. 13, 1961 3 Sheets-Sheet. 1

8 o o o o o o o o 0 o 0 o o 9* We) 0 o o 0 INVENTORS c. s. HALL, JR, W. B. HUCKABAY, BY M.C.KE| SEY mwa fl wwz ATTORNEYS A ril 27, 1965 Filed Dec. 13, 1961 6A5 ANALYZER NO. OF SAMPLES c. s. HALL, JR., ETAL 3,180,983

GEOCHEMICAL PROSPECTING 3 Sheets-Sheet 2 PA C/(ER GAS NO. OF SAMPLES PPM ME THA ,4/5

INVENTORS as. HALL, JR, W. B. HUCKABAY, BY M. c. KELSEY QW,MW

ATTORNEYS April 27, 1965 c. s. HALL, JR, ETAL 3,180,983

GEOCHEMICAL PROSPECTING Filed Dec. 13, 1961 3 Sheets-Sheet 3 ROAD) Lfi (f w 5] 10x 10% 10:4 10;: E [E [5' El STATIONS ---v s: 1o ax ROADS 10x 15% 0x 5: 1oz

J \j x 2;; I5 X ATTORNEYS United States Patent .This invention relates generally to an improved method and apparatus for locating subterranean petroleum deposits, and more particularly, but not by way of limitation, to an improved method of prospecting for subterranean petroleum deposits involving sampling soil gas at or near the surface of the earth.

As it is well known in the exploration division of the oil industry, many eiforts have been made to obtain a direct indication of the location of subterranean petroleum deposits by means of measuring the methane content of soil gas at or near the surface of the earth. In the more elementary method of such prospecting, a plurality of rather shallow holes are drilled in a random fashion over an area being investigated and the soil gas in each of the holes is sampled or analyzed for methane content. Many wells have been drilled at the location of a sampling point which gave the highest methane content reading, and, in a few isolated cases, oil has been found by such a method. However, this type of method is not reliable for various reasons. Perhaps the primary reason is that methane gas is .given off by decaying organic material, and the sample relied upon for locating a well is just as likely to have been influenced by decaying organic material as by methane being given 01f by a petroleum deposit. Another factor affecting the reliability of such a single sample type of method is that varying climatic conditions and varying soil conditions have a large effect on the analysis of soil gas at any particular location.

That is, the methane content of soil gas obtained from a particular sampling hole will vary drastically with changes in climatic conditions, such as a change from clear to rainy weather. Also, it has been found that the methane content of soil gas obtained from holes drilled in two ditferent types of soils will vary greatly even though both test holes may overlie the same oil bearing 0r non-oil bearing formation.

One rather well-known theory attempting to explain the appearance of methane gas at or near the surface of the earth overlying a petroleum deposit is that the oil or gas migrates from a deposit by a process of diffusion. Another theory is that oil from a deposit or reservoir migrates through minute cracks and crevices and reaches the surface of the earth through a multitude of rather distinct paths which extend like fingers in various directions from the oil deposit. It will be readily seen that locating a well in accordance with the maximum methane reading obtained from a plurality of test holes would only, by coincidence, result in a finding of the oil deposit. This is particularly true with respect tothe second theory, since the particular sample relied upon may have been obtained precisely at the'upper end of one of the fingers extending at a rather large angle to the vertical from the oil deposit, such that a well drilled straight down in spaced relation over the area being investigated. The present method is based on the laws of probabilities involving the distribution of random statistical data, as is explained in detail below. Thus, the present method is based'up'on odds and, in order to provide a practical data handling operation, the number of holes drilled or samples taken at each station location preferably coincides with the odds selected for the simplest calculations.

The same number of test holes are drilled at each station location, but the stations need not be precisely or uniformly distributed over the. area being investigated. We have found that the stations may be located in the most accessible portions of the area being investigated, so long as the stations are sufiiciently distributed to represent a cross-section of the soil gas obtained over the entire area. The methane content of the soil gas obtained from each of the test holes is determined and recorded, usually immediately following the drilling of the test holes at a particular station. The methane readings are then plotted in a distribution curve, or histogram, in order that the standard deviation of the various methane readings can be determined. We have found that the leading side of the histogram may be used for this purpose, since the trailing half of the histogram will not be symmetrical with the leading half if the area overlies or partially overlies an oil deposit. The methane readings at the individual stations are then analyzed to determine how many methane readings at a particular station exceed the selected number of standard deviations. We then plot the number of readings at the various stations which exceed the selected standard deviations and contour the data on the map. With this method, the area enclosed by the highest values is considered the area overlying an oil deposit and determines the well location.

An important object of this invention is to provide a method and apparatus for directly locating subterranean petroleum deposits.

Another object of this invention is to provide a method of locating subterranean petroleum deposits which is simple and economical to perform.

Another object of this invention is to provide a method of locating subterranean petroleum deposits involving a sampling of soil gas at a plurality of points over an area being investigated which involves a minimum of data handling. V

A further object of this invention is to provide a method of petroleum exploration which involves a minimum damage to the area being investigated.

Another object of this invention is to precisely determine the methane content of soil gas.

Another object of this invention is to provide a simple method and apparatus for obtaining a soil gas sample.

A further object of this invention is to provide a method of obtaining a plurality of soil gas samples for methane analysis, wherein the samples are taken in a minimum of time and the samples are not contaminated by gases in the atmosphere.

A still further object of this invention is to provide an economical method of exploring for petroleum deposits which obtains ahigh degree of precision in distinguishing between barren and oil bearing areas.

Other objects and advantages of the invention will be evident from. the following detailed description, when read in conjunction with the accompanying drawings which illustrate the invention.

In the drawings: FIGURE 1 is a schematic drawing in the nature of a map of an area being investigated by the present inven- FIGURE 3 is a schematic view of the apparatus utilized in taking and analyzing a soil gas sample.

FIGURE 4 is a typical curveof the variation in the methane content of a soil gas sample.

FIGURE 5 is an illustration of a conventional distribution curve or histogram illustrating the change in the curve upon the occurrence of an oil bearing area.

FIGURE 6 is a histogram in the form of a bar graph of the type utilized in the present method.

FIGURE 7 is aview similar'to FIGURE 1 illustrating a typical contour of the data obtained at the various stations.

Referring to the drawings in detail, and particularly FIG. 1, reference character 10 designates each of a plurality of sampling stations arranged in spaced relation over an area to be investigated or explored for a subterranean petroleum deposit. As previously indicated, the stations 16 need not be precisely located in accordance with any predetermined geometrical pattern over the area being investigated, it only being necessary that the stations be arranged in spaced relation and distributed over the area being investigated. In a typical exploration job, the stations 10 are located adjacent roads running through or over the area being investigated and are spaced apart at approximately one eighth to one quarter of a mile. This variation in spacing will allow the stations 11 to be located in places where test holes can be easily drilled without the necessity of cutting any appreciable amount of brush or trees or otherwise clearing the land for the testing procedure. It is necessary, however, that the locations of the stations 11) be precisely plotted on a map of the area being investigated, as illustrated in FIG. 1. i

The spacing of the stations 11) referred to above is the desired spacing for a detailed survey of an area. When a large area is to be investigated, such as an area of several square miles, we can locate stations a rather large distance apart, such as in each section of the area, to first determine which portion of the large area should be investigated. Also, after an area has been investigated and shows promise, additional test holes are drilled in the most interesting portion of the area to further deiineate the anomalies, as will be described.

As illustrated in FIG. 2, the test holes 12 at each station 111 are preferably located in spaced relation in a pattern of rows and columns to uniformly cover the station location. In a preferred embodiment, and to greatly facilitate the handling of test data, twenty holes '12 are drilled at each station 10. The selection of twenty test holes 12 at each station is made to correspond to the odds against the occurrence of a methane reading exceeding two standard deviations with respect to all of the test readings obtained, as will be described more in detail below. The various holes 12 are drilled in any suitable manner, such as by means of a conventional post hole digger, either manual or power driven, and each hole 12 is preferably drilled anywhere from two to four feet deep. In a normal procedure, all of the test holes 12 at a station lti are drilled one after another and then the drilling crew moves on to the next station location while the testing crew takes soil gas samples from the newly drilled holes 12. With this procedure, the soil gas is not contaminated by exhaust gases from the vehicle or vehicles employed by the drilling crew.

A soil gas sample is taken from each test hole 12 by use of a novel, yet extremely simple, probe 14 illustrated in FIG. 3. The probe 14 comprises a large diameter pipe 16 telescoped over a smaller diameter pipe 18, with the large diameter pipe 15 being shorter than the pipe 18. The upper end of the large diameter pipe 16 is secured to, and sealed around, the smaller diameter pipe 18 by a suitable seal structure 20; and the lower end 22 of the pipe 16 is sealed around the smaller pipe 18 by an inflatable packer 24. The packer 24 simply comprises a tubular rubber member secured at its upper end to the lower end 22 of the pipe 16 and secured at its lower end around the lower end portion 26 of the pipe 18, with the opposite ends of the packer 24 being sealed to the respective pipes 16 or 18 in gas-tight relation. The packer 24 is inflated by any suitable gas fed thereto through an inlet pipe 28 and through the larger diameter pipe 16 around the smaller diameter pipe 18. In one embodiment of this invention, the packer 24 is formed out of a section of an inner tube and the gas pressure utilized to inflate the packer is approximately 15 psi. Any suitable gas can be used, such as air or nitrogen.

The size of the sampling device 14 will obviously depend upon the sizes of the test holes 12. In one embodiment of this invention, the test holes 12 are drilled anywhere from two to four feet deep with a conventional post hole digger and the sampling device 14 is of a size to be easily inserted in each test hole. The device 14 is supported in a test hole 12 with the lower end 26 of the smaller diameter pipe 18 spaced slightly above the bottom of the test hole by means of a pair of arms 30 secured to opposite sides of the pipe 16 by suitable hinges 32. The hinges 32 are constructed to allow the arms 39 to fold downwardly alongside the pipe 16 for ease in transporting the device. However, when the arms 31) are swung upwardly to the positions illustrated in FIG. 3, the inner ends of the arms contact the opposite sides of the pipe 16 and prevent further upward movement of the arms. Thus, when the arms 30 are resting upon the surface 34 of the ground, the device 14 will be supported in the respective test hole 12 with the lower end 26 of the pipe 18 spaced upwardly from the bottom of the hole.

Soil gas is withdrawn from the test hole 12 by means of a suitable air pump 36 connected to the upper end of the pipe 18 by a suitable hose 38. The discharge of the pump 36 is connected to a suitable gas analyzer 40 to force the soil gas sample through the analyzer and measure the methane content of the soil gas. It may also be noted here that when an infrared type of gas analyzer til is utilized, the pump 36 produces a suction on the pipe 18 of approximately two ounces, compared with the approximately fifteen lbs. gas pressure applied inside the packer 24. With this arrangement, the packer 24 effectively excludes atmospheric gases from flowing downwardly along the Walls of the test hole 12 and into the lower end 26 of the pipe 18. Thus, the gas removed from the lower portion of the hole 12 is representative of gas contained in the soil around the test hole. It may also be noted here that the analyzer 40 is provided for the sole purpose of measuring the methane content of the soil gas. When the analyzer 49 is in the form of an infrared type analyzer which utilizes an infrared beam passing through the sample being measured, we have found it highly useful to insert a test cell containing wet carbon dioxide in the infrared beam prior to passage of the beam through the sample being measured. With this arrangement, the gas in the test cell effectively absorbs those portrons of the infrared beam affected by both water and carbon dioxide, such that the absorption of the beam in the test cell containing the soil gas sample will be affected only by the methane content of the sample.

FIG. 4 is provided to illustrate a typical soil gas methane analysis curve. It will be observed that the starting portion 42 of the curve is relatively fiat for an appreciable period of time at a level between zero and five parts per million (p.p.m.) of methane. The portion 42 of the curve represents the analysis of the gas standing in the pipe 18, hose 38 and pump 36 when the device 14 is first inserted into a test hole 12. It will also be noted that a this will be substantially atmospheric gas and such gas will normally have a methane content of from two to four ppm. As the actual soil gas from the test hole 12 enters the analyzer 40, the curve turns upwardly and reaches a normally very distinct peak 44 which is submediatelyadjacent the test hole; whereas the level or fiat curve shown in FIG. 4 is typical of the curve obtained from a large number of test holes utilized in the practice of this invention. It may also be noted that when determining the methane content of the soil gas from each test hole 12, either the value provided by the peak 44 or the value provided by the portion 46 of each curve may be utilized, as long as the same procedure is utilized for each sample obtained in a surveying operation. That is, all of the methane content values should be obtained from the peaks 44 of all the curves for the soil-gas samples, or all of the methane contents should be obtained fro the portions 46 of all the samples.

When the methane contents of all the samples are plotted against the number of samples, as illustrated in FIG. '5, the plotted data provides a curve of the distribution of the methane contents, which is commonly known as a histogram. We have found that regardless of whether the samples are taken from an area which overlies barren formations or oil bearing formations, the

leading half 48 of the histogram is in the usual form of a histogram. If the area overlies barren formations, the trailing half of the curve is symmetrical with the leading half of the curve as indicated by the dashed line 50 in FIG. 5. However, if the area being investigated overlies or partially overlies an oil bearing formation, the trailing half of the histogram is not symmetrical with the leading half 48 as indicated by the solid curve 52. Thus, a nonsymmetrical distribution curve is indicative of an oil bearing formation below the area'being investigated, and the problem lies in determining the precise location of the oil bearing formation. This determination is made by application of the laws of probabilities which we have found is greatly facilitated by plotting the samples in the form of a bar graph histogram as illustrated in FIG. 6.

As previously described in connection with FIG. 4, the initial portion 42 of the methane analysis curve clearly indicates that atmospheric gases contain a definite portion 'of methane which normally varies between two and four p.p.m. In order to eliminate the methanecontent of atmospheric gases, the initial portions 42 of the methane analysis curve are disregarded. Zero on the p.p.m. scale of the bar graph in FIG. 6 is established bypassing dry nitrogen (previously tested) through the analyzer '40 immediately prior to submitting the soil gas withdrawn'from the test hole 12. i

FIG. 6. The graph further shows that there are fifty samples having approximately 0.4 p.p.m. methane, etc.

As previously stated, the leading half of a distribution curve obtained from the methane'analysis of soil gas samples is representative of the leading half of a normal distribution curve; whereas the trailing half of the curve will not be representative of a normal distribution curve if the area being investigated overlies an oil deposit. We therefore utilize the leading half of the curve provided by the bar graph in FIG. 6 for determination of the standard deviation. By assuming that the trailing half of the distribution curve is identical to the leading half, the distribution curve so constructed is symmetrical andwith a symmetrical distribution curve the mode (incremental value of most occurrence), the median (the mid point of value), and the arithmetic mean is the same value. To determine standard deviation the arithmetic mean must be used, but it is determined from this constructed symmetrical distribution curve by making the mode of the actual curve, FIG. 6, the arithmetic mean of the constructed curve.

According to the laws or probabilities, the standard deviation is the square root of variance; wherein variance is the sum of the deviations of the individual measure- We plot the methane contents thus recorded against the methane values nearest 0.2 p.p.m. are plotted as a group, there being thirty such samples in the graph illustrated in graph or curve, the mode is easily observed and visually determined. In the example shown in FIG. 6, the mode,

and hence the mean, is 1.20 p.p.m. The standard deviation is determined by. the following steps:

(a) Calculate the difference between the mean and the lowest incremental group:

(b) Square the diiference determined by Step [1:

(0) Multiply the sum obtained by Step b by 2:

(d) Multiply the results obtained by Step 0 by the number of samples in the lowest incremental group:

(e) Repeat steps a through ,d for each of the higher incremental groups which would be the groups at 0.4, 0.6, 0.8 and 1.0 in the example shown in FIG. 6:

(7") Add the results obtained in Step d for each of the incremental groups:

(g) Divide the sum obtained by step f by 2 times the number of samples in the lowest incremental group, plus two times the number of samples in the next incremental group, etc., plus the number of samples in the mean group:

(h) Take the square root of the quotient obtained from Step g:

In accordance with the present invention, reference is next made to a table of the Probability of Occurrence of Deviations, such as is contained on page 208 of the Handbook of Chemistry and Physics, 37th edition, by Chemical Rubber Publishing Co., published in 1955-1956. This table shows the odds against the occurrence of any particular value of a series as the magnitude of the value is related to the standard deviation of the series. For example, the odds are 79.52 to 1 that a particular value of a series will not exceed 2.5 standard deviations. Such a relationship is applied in the present method to correlate the methane readings at the various stations and determine which, of the stations overlie an oil-bearing formation. In other words, we analyze the methane readings separately at each station '10 and find the percentage of such readings which exceed the number of standard deviations corresponding to the odds selected for analyzing the data.

It will be apparent that the greater the odds used, the

more precise will be the results obtained. However, the odds selected must correspond or substantially correspond with the number of samples taken at a particular station, since no sample is divisible. For example, if the odds selected are 1,033, there must be 1,033 samples taken at a station, or the number of samples exceeding the corresponding 3.3 standard deviations cannot be determined by the laws of probabilities. However, the number of samples which can be taken at a station is, as a practical matter, limited, and the precision in calculations need be no more than the precision of the methane content measnrements. We therefore prefer to use odds of :1 and take twenty samples at each station 10.

In examining the above mentioned table, it will be found that odds of 20:1 are not listed. However, the odds are 20.98c1 that a particular value will not exceed two standard deviations, and these odds are the closest listed'to 2011. Therefore we analyze each of the methane readings at each station 10 to determine the percentage exceeding two standard deviations, and the analysis is sound since this procedure merely requires a slightly higher methane value for a particular reading to be counted than would be required by strict adherence to the laws of probabilities.

We also prefer to utilize twenty samples at each station 10 because this number of samples takes into account the probable percent error incurred in obtaining the methane values. The percentage of methane readings at a station exceeding the required two standard deviations is determined by dividing the number of those readings by twenty (such as 4/ 20:20% It will thus be seen that the percentages are determined in'increments of 5% which is quite suflicient to take into account testing and analysis errors.

After the standard deviation has been determined, the methane readings at all of the stations 10. are analyzed and each station is given a percentage value depending upon the number of readings at each station exceeding the required standard deviations corresponding to the odds and number of samples being used.

The station percentage values are then plotted on the station locations on a map of the area being investigated as illustrated in FIG. 7. It will be understood that the percentage figures used in FIG. 7 are for illustration only and do not necessarily correspond to the figures obtained in any particular survey. However, the percentage figures are always, as previously indicated, in increments of 5% when twenty samples are taken at each station.

After the percentage values are plotted on the map, the corresponding values are contoured, as also illustrated in FIG. 7. In accordance with the present invention, the highest values contoured in FIG. 7) define the area overlying a petroleum deposit and the well to be drilled is located in this area. It will be understood that if the area enclosed by the highest values is of appreciable size, such 8 as several acres, an additional survey is run in such area in accordance with the method described above to more accurately locate the Well to be drilled.

The method described above is, as indicated, the method to follow for an area of somewhat limited area, such as a section of land. If a large area, such as several sections, is to be surveyed to determine which portion deserves detailed consideration, stations 10 are located a substantial distance apart, such as one or two stations to a section. With this exception, the method outlined above is followed, and the resulting area enclosed by the highest contoured values is subsequently surveyed in the manner outlined.

As previously noted, any appreciable change in climatic or soil conditions sometimes provides a drastic change in the specific methane values obtained from test holes overlying the same formations. Therefore, with a change in either of these conditions, the methane values cannot be plotted into a single histogram.' We have found that if the weather changes, say from sunshine to rain, while making a survey, or if the top soil in the area being investigated changes appreciably, say from a sandy loam to clay, two or more histograms must be utilized.

Assuming that a survey was half completed on a clear day and then the last half of the test holes 12 drilled and the samples taken and measured during rain. In this event, the first half of the methane values obtained are plotted in one histogram and the subsequently obtained values plotted in a separate histogram. The standard deviations of the separate histograms are determined, and the methane readings at the individual stations are analyzed with respect to the corresponding histograms. For example, the methane readings obtained at the first station 10 are analyzed with respect to the standard deviation obtained from the first histogram and the methane readings obtained at the last station 10 are analyzed with respect to the second histogram. We have found that with this procedure, all of the percentage values plotted on the station locations on the map of the area can be contoured in the same way as when all of the methane contents are obtained under very similar conditions. The same procedure applies to samples obtained from varying soils.

From the foregoing it will be apparent that the present invention provides a novel method and apparatus for directly locating the position of a subterranean petroleum deposit. The method causes no appreciable damage to property since no explosives are used and the shallow test holes can be easily covered if necessary. The method involves a minimum of data handling and yet the precision obtained in the data handling is as accurate as, and takes into consideration the inherent errors in, the sample taking and methane measurements. It will further be apparent that the apparatus required is simple and the method is easily and economically performed.

Changes may be made in the combination and arrangement of steps and procedures, as well as in the various elements of the apparatus, without departing from the spirit and scope of the following claims.

What is claimed is:

l. A method of locating subterranean petroleum deposits, comprising the steps'of:

(a) drilling a plurality of test holes at each'of a plurality of stations located in spaced relation over the area being investigated;

(b) withdrawing a sample of soil gas from each hole;

(0) measuring and recording the percentage of methane in each soil gas sample;

(d) determining the mean of all the methane values;

(:2) plotting the locations of the stations on a map of the area being investigated;

(f) plotting on each station location .on the map the percentage of samples at the respective station having a methane value exceeding the mean by a predetermined minimum; and

(g) contouring the values obtained in step 1'1, whereby the area closed by the highest values indicates the all the samples, said histogram having a leading and i a trailing half;

(e) calculating the standard deviation of the histogram using the leading half of the histogram;

(f) plotting the locations of the stations on a map of the area being investigated;

(g) plotting on each station location on the map the number of samples at the respective station which have a methane content exceeding at least one standard deviation; and

(h) contouring the numbers plotted on the map, whereby the highest closed values indicate the area overlying the petroleum deposit.

3. A method of locating subterranean petroleum deposits, comprising the steps of:

(a) drilling a plurality of test holes at each of a plurality of stations located in spaced relation over the area being investigated, the number of holes at each station being equal; a

(b) withdrawing a sample of soil gas from each hole;

() measuring and recording the methane content of each sample;

(d) plotting a histogram of the methane contents of all the samples, said histogram having a leading and a trailing half; a

(e) calculating the standard deviation of the histogram using the leading half of the histogram;

(f) plotting the locations of the stations on amap of the area being investigated;

(g) plotting on each station location on the map the number of samples at the respective station which have a methane content exceeding the median of all the samples plus the number of standard deviations corresponding to the odds of X:1 as determined by the law of probabilities, wherein X approximately equals the number of holes drilled at each station; and,

(h) contouring the numbers plotted on the map, Whereby the highest closed values indicate the area overlying the oil deposit.

4. A method as defined in claim 3 wherein twenty holes are drilled at each station.

5. A method as defined in claim 2 wherein the soil gas samples at each station are taken and the methane content measured and recorded immediately following the drilling of the holes at the respective station.

6. A method as defined in claim 2 wherein the histogram is plotted in bar graph form. I

7. A method as defined in claim 6 wherein the methane contents are plotted in the histogram to the nearest 0.2 parts per million.

8. A method as defined in claim 1 wherein the percentage of methane is measured by passing an infrared beam through the sample, and characterized further to include the step of passing the infrared beam first through a body 1% of wet carbon dioxide to eliminate the effects of water and carbon dioxide in the soil gas sample being measured.

9. A method of locating a subterranean petroleum deposit during varying climatic conditions, comprising the steps of:

(a) drilling a plurality of test holes at each of a plurality of stations located in spaced relation over the area being investigated, the number of holes at each station being equal;

(b) withdrawing a sample of soil gas from each hole;

(0) measuring and recording the methane content of each sample; I

(d) plotting a separate histogram of the methane content of each series of samples obtained under similar climatic conditions, each said histogram having a leading and a trailing half;

(e) calculating the standard deviation of each histogram using the leading half of the histogram;

(f) plotting the locations of the stations on a map of the area being investigated; 7

(g) plotting on each station locationon the map the percentage of samples at the respective station which have a methane content exceeding at least one standard deviation of the corresponding histogram; and

(h) contouring the percentages plotted on the map, whereby the highest closed values indicate the portion of the area overlying the petroleum deposit.

10. A method of locating a subterranean petroleum deposit underlying an area having varying soil compositions, comprising the steps of:

(a) drilling a plurality of test holes at each of a plurality of stations located in spaced relation over the 7 area being investigated, the number of holes at each station being equal;

i (b) Withdrawing a sample of soil gas from each hole;

(c) measuring and recording the methane content of each sample;

(d) plotting a separate histogram of the methane content of each series of samples obtained from similar soil compositions, each said histogram having a leading and a trailing half;

(e) calculating the standard deviation of each histogram using the leading half of the histogram;

(f) plotting the locations of the stations on a map of the area being investigated;

(g) plotting on each station location on the map the percentage of samples at the respective station which have a methane content exceeding at least one standard deviation of the corresponding histogram; and

(h) contouring the percentages plotted on the map,

whereby the highest closed values indicate theportion of the area overlying the petroleum deposit.

References Cited by the Examiner UNITED STATES PATENTS RALPH G. NILSON, Primary Examiner. ROBERT EVANS, Examiner.

McDennott 23-230 

2. A METHOD OF LOCATING SUBTERRANEAN PETROLEUM DEPOSITS, COMPRISING THE STEPS OF: (A) DRILLING A PLURALITY OF TEST HOLES AT EACH OF A PLURALITY OF STATIONS LOCATED IN SPACED RELATION OVER THE AREA BEING INVESTIGATED, THE NUMBER OF HOLES AT EACH STATION BEING EQUAL; (B) WITHDRAWIONG A SAMPLE OF SOIL GAS FROM EACH HOLE; (C) MEASURING AND RECORDING THE METHANE CONTENT OF EACH SAMPLE; (D) PLOTTING A HISTOGRAM OF THE METHANE CONTENTS OF ALL THE SAMPLES, AND HISTOGRAM HAVING A LEADING AN A TRAILING HALF; (E) CALCULATING THE STANDARD DEVIATION OF THE HISTOGRAM USING THE LEADING HALF OF THE HISTOGRAM; (F) PLOTTING THE LOCATIONS OF THE STATIONS ON A MAP OF THE AREA BEING INVESTIGATED; (G) PLOTTING ONE EACH STATION LOCATION ON THE MAP THE NUMBER OF SAMPLES AT THE RESPECTIVE STATION WHICH HAVE A METHANE CONTENT EXCEEDING AT LEAST ONE STANDARD DEVIATION; AND (H) CONTOURING THE NUMBERS PLOTTED ON THE MAP, WHEREBY THE HIGHEST CLOSED VALUES INDICATE THE AREA OVERLYING THE PETROLEUM DEPOSIT. 